Light of any nature, including continuous wave and pulsed light (hereinafter collectively referred to as light beams), comprises shot noise. Shot noise is quantum noise. Shot noise exists because the rate of photons in a light beam is not uniform but is random to a certain extent. The smallest increment with which the phase or amplitude of a light beam can be determined, i.e., the accuracy of phase or amplitude determination, is limited by shot noise. For example, in a fiber optic interferometer in which two light pulses are compared to each other by a balanced detector after they counter propagate through an optical fiber loop, the accuracy of the balanced detector in detecting a phase difference between the two pulses is limited by the shot noise of the system.
Optical squeezing is a method for reducing the effect of shot noise. When light is introduced into an optical fiber at very high intensity, i.e., on the order of 1 kilowatt or greater, the index of refraction of the fiber varies slightly for different intensities. This difference in index of refraction causes the speed at which photons of different intensities travel through the fiber to be different, resulting in relative phase shifts for light pulses of different intensity.
In an optical fiber in which squeezing is not occurring, the probable phase and amplitude of an intense light beam follows a generally gaussian distribution. The amplitude distribution of the vacuum state would be generally circular, as shown by circle 11a in the phasor diagram of FIG. 1A, and the field of the light could be anywhere within the circle. If the light had an amplitude of X and a phase of .THETA. then the phasor diagram would be as shown in FIG. 1B and the field of the light may be anywhere within circle 11b. However, in an optical fiber experiencing squeezing, the probable phase and amplitude of light is altered due to nonlinear light effects in the fiber.
The phasor diagram of a squeezed vacuum is elliptical, as shown at 13a in FIG. 2A. FIG. 2B illustrates the situation for squeezed light of amplitude X and phase .THETA. at 13b. The orientation of the major axis of the ellipse is a function of the phase shift. A light beam which has a phase .THETA. such that it is oriented generally parallel to the minor axis of the ellipse, such as vector 25 in FIG. 2B, has less quantum noise in the photon number than unsqueezed light, i.e., sub-shot noise. If, on the other hand, the phase of the light was oriented generally parallel to the major axis of the ellipse, as illustrated by vector 27 in FIG. 2C, that light would have more quantum noise in the photon number than unsqueezed light.
Reference can be made to Bergman, K. and Haus, H. A., Squeezing in Fibers With Optical Pulses, Optics Letters, vol. 15, No. 9, May 1, 1991, as well as Shirasaki, M. and Haus, H. A., Squeezing of Pulses in a Non-Linear Interferometer, J. Opt. Soc. Am., vol. 7, No. 1, January 1990 for more thorough discussions of squeezing in optical fibers.
Further, a type of interferometric detection scheme using squeezed light is suggested in Shirasaki, M. and Haus, H. A., Non-Linear Guided-Wave Phenomena: Physics and Applications, vol. 2, OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1989), p. 232, and Shirasaki, M. and Haus H. A., Journal of the Optical Society of America, B7, 30 (1990).
Guided-acoustic-wave Briliouin scattering (GAWBS) noise is a noise imparted to a light beam by thermal vibration in an optical fiber. GAWBS generally occurs at very high frequencies, on the order of 20 MHz-1 GHz. The thermal vibration of GAWBS noise alters the index of refraction of the fiber. Light pulses in different parts of the fiber will be subject to different GAWBS noise and, therefore, different indices of refraction, thus introducing a phase shift between different pulses. In applications where the relative phase of two distinct light pulses is of significance, such as a fiber optic interferometer, GAWBS noise reduces phase shift measurement accuracy.
GAWBS noise is a significant problem in squeezing since the light must pass through an optical fiber loop in which GAWBS noise is present in order for squeezing to occur. As the length of the fiber increases, GAWBS noise also increases. Accordingly, it has been proposed to decrease GAWBS noise through use of a short fiber and extremely short pulses with high peak power. Shelby, R. M., Levenson, M. D. and Bayer, P. W., Phys. Rev. B. 31, 5244 (1985). Further, Sakai, Y., Hawkins, R. J. and Friberg, S. R., Soliton-Collison Interferometer for the Quantum Nondemolition Measurement of Photon Number: Numerical Results, Optics Letters, 15, 239 (1990) suggest reducing GAWBS noise in soliton-collision photon number measurements by spacing the probe soliton (which is collided with a signal soliton) within a time interval that is small compared with the inverse spectral width of the GAWBS noise of a reference soliton and interferometrically detecting the phase shift between the collided probe soliton and the uncollided reference soliton. Since the two solitons are spaced apart by an interval which is small compared with the inverse spectral width of GAWBS noise, both pulses experience the same GAWBS noise and, therefore, any phase shift caused by GAWBS noise shows up in both the probe and reference soliton and is, therefore, cancelled in the comparison.
It is an object of the present invention to reduce GAWBS noise in an optical squeezer.
It is a further object of the present invention to provide an optical interferometer utilizing squeezed light having reduced GAWBS noise.